This invention relates to a method of forming a thin film on a substrate by the reactive DC sputtering.
It is usual to use oxygen gas as a reactive gas and argon gas as an inert gas for a conventional reactive DC sputtering. Hitherto, a conventional reactive DC sputtering has been conducted through a constant electric power or a constant electric current. However, in the conventional reactive DC sputtering, a reaction film tends to be formed on a metal target surface during sputtering through the reaction between the reactive gas and the metal of the target. This reaction film lowers the sputtering rate and induces the occurrence of abnormal discharge. Due to the occurrence of abnormal discharge, impurities tend to stick to a substrate surface. With this, the external appearance of the film and the film characteristics tend to be deteriorated.
JP-A-64-79369 proposes a sputtering method in which sputtering power input is increased substantially exponentially from the beginning of sputtering as a film accumulates on a substrate. The sputtering rate is increased by this method. However, discharge does not always become stable and the maximum sputtering rate can not always be maintained by this method.
It is generally known in a reactive DC sputtering that a reaction film is formed on a metal target surface, and that the maximum discharge voltage which can be applied between a target and a substrate fluctuates. Due to this fluctuation, the sputtering rate fluctuates, too. The maximum discharge voltage corresponds to the maximum sputtering rate. That is, when the discharge voltage is at the maximum, the sputtering rate becomes maximum, too. Discharge becomes stable at the maximum or nearly maximum sputtering rate. It should be noted that the maximum or nearly maximum sputtering rate can not always be maintained by the above-mentioned constant power input and the exponentially increasing power input.